Sixth seed Andy Roddick tamed powerful Croatian Mario Ancic 6-3, 3-6, 6-1, 5-7, 6-4 in a pulsating match to reach the Australian Open quarter-finals on Sunday.

The American, who beat former champion Marat Safin in the last round, took just over three hours to seal his fifth successive victory over the ninth seed and advance to the last eight at Melbourne Park for the fourth time.

Serena Williams continued her role of loose cannon in the women's draw by shocking in-form 11th seed Jelena Jankovic 6-3, 6-2 in the fourth round.

The unseeded two-times former champion put on a formidable display of hitting and phenomenal chasing to overcome the fancied Serb in 83 minutes on Rod Laver Arena.

"I like being a dangerous floater," Serena, who is currently ranked 81st, told the crowd.

The 25-year-old American, champion in 2003 and 2005, will now play Israel's 16th seed Shahar Peer for a place in the semi-finals.

Roddick, again supported from the stands by his coach Jimmy Connors, romped through the first set in 27 minutes but the 22-year-old Ancic hit back to break Roddick's mighty serve and level the fourth-round clash.

Roddick, a former world number one and US Open champion, kept his composure to break Ancic twice in the third set but the Croatian rallied in the fourth, which was interrupted by a short rain delay.

"It was a pretty good match with a pretty high level from both us," Roddick told a news conference. "That was definitely the best Mario has played against me."

"The match against Marat was probably a bit more physical in terms of rallying but I felt really good out there today."

His raking returns set up a break of serve and he sealed the set it with a brilliant lunging forehand down the line. The fifth set went with serve until Ancic cracked, pushing a loose backhand volley wide of the line to edge Roddick 3-2 ahead.

The American stayed solid on his serve to close out victory on his second match point in just over three hours.

Connors only arrived in Australia on Friday after missing Roddick's first two matches in the tournament to stay in the US following the death of his mother.

The 24-year-old Roddick said his coach had not bombarded him with advice after the Safin match.

"Jimmy is pretty concise, he's not big on the war room or sit down heart to heart," Roddick said. "But if he sees something he thinks I didn't do well then he tells me."

Roddick, a semi-finalist at Melbourne Park in 2003 and 2005, will play fellow American Mardy Fish, a good friend who attended the same high school, in the last eight.

"It's a little bit weird for us to play each other in a major because of our history," Roddick said. "But we are both professional enough to know it's a big match so it should be fine."

Roddick has won four of their five career meetings with Fish's only victory coming in their first match in 2003.